1. Field of the Invention
One embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the invention disclosed in this specification and the like relates to a process, a machine, manufacture, or a composition of matter.
In particular, one embodiment of the invention disclosed in this specification and the like relates to a semiconductor device and an electronic device including the semiconductor device.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device (e.g., a liquid crystal display device and a light-emitting display device), a lighting device, an electro-optical device, a power storage device, a memory device, a semiconductor circuit, an imaging device, an electronic device, and the like may include a semiconductor device.
2. Description of the Related Art
With the widespread of large display devices such as liquid crystal televisions, higher value added products have been demanded and thus under development. In particular, a technique to configure a driver circuit such as a scan line driver circuit over the same substrate as a pixel portion, using thin film transistors (TFTs) whose channel regions are made of an amorphous semiconductor has been actively developed because the technique greatly contributes to reduction in cost and improvement in reliability.
A thin film transistor whose channel region is formed using an amorphous semiconductor causes deterioration such as an increase in threshold voltage or a decrease in field-effect mobility. If deterioration in the thin film transistor proceeds, there arises a problem such that a driver circuit may have difficulty in operation and may fail to display an image. Therefore, a shift register capable of suppressing deterioration in a thin film transistor is disclosed in Patent Document 1. In Patent Document 1, in order to suppress degradation of thin film transistor characteristics, two thin film transistors are provided and the thin film transistors are connected between an output terminal of a flip-flop and a wiring to which VSS (hereinafter referred to as negative power supply) is supplied. Moreover, one thin film transistor and the other thin film transistor are alternately turned on. In such a manner, a period during which each of the thin film transistors is on can be shortened by approximately half in a frame period; therefore, degradation of characteristics of the thin film transistors can be suppressed to some extent.